The present invention departs from needs which have been recognized at behind-the-ear hearing devices, thereby especially at behind-the-ear hearing aid devices. Nevertheless, although departing from such devices, the present invention may be applied to all hearing systems where there is applied a microphone arrangement not within the ear canal, thereby especially behind the pinna of an individual's ear, the output of which operationally acting on an electrical to mechanical output converter which is applied to the same and/or the other ear of the individual. The hearing device may be a device for increasing hearing capability or a hearing protection device.
Today's behind-the-ear hearing devices and thereby especially behind-the-ear hearing aid devices may controllably be enabled to operate in the so-called omnidirectional mode. The microphone arrangement which, for this mode, may consist of one omnidirectional microphone, provides per se for an omnidirectional transfer characteristic, which means that acoustical signals impinging on the microphone arrangement are converted to an electrical output signal with a predetermined constant amplification irrespective of the direction with which such acoustical signals impinge on the arrangement. Nevertheless, once applied adjacent to the top of or behind the pinna, the acoustical to electrical transfer characteristic becomes not anymore independent of the direction at which acoustical sources appear to the microphone due to the so-called head-related transfer function HRTF, which results in some degree from “shadowing” of the acoustical signals dependent where the acoustical signal source is located with respect to the microphone arrangement.
When an individual's ear acoustical reception characteristic is investigated per se, e.g. by means of a complete-in-the-canal hearing device, CIC, as a standard the following transfer characteristics are recognized:
When, according to FIG. 1, 0° direction is defined in the direction at which individual's head H faces and 90° direction is defined perpendicularly thereto in a direction pointing outwards of individual's right ear ER one recognizes at this right ear a transfer characteristic at a frequency f of 0.5 kHz of acoustical signals as shown in FIG. 2. As may be seen the acoustical signals from acoustical sources seen under an angle of 180° to 0° are considerably attenuated, which is predominantly caused by the shadowing effect of individual's head, i.e. by the HRTF. In directions symmetrically to the 90° directivity axis, i.e. at about 45° and at about 135°, the amplification is substantially equal.
In FIG. 3 as well as in FIG. 4 the respective transfer characteristics are shown for acoustical signals at 1 kHz and at 2 kHz.
In FIG. 5 the transfer characteristic is shown at 4 kHz. When comparing the transfer characteristics at 2 kHz, with that at 4 kHz, according to the respective FIGS. 4 and 5, one recognizes an increased directivity of the transfer characteristics at 4 kHz. In fact, departing approx. at frequencies of 2 kHz of the acoustical signals, the directivity of the pinna becomes effective. Nevertheless, as frequencies above 7 kHz are of no interest whenever speech understanding is addressed, with an eye on such speech understanding, it is important to note that the pinna provides for a beam forming effect in a frequency band of 2 kHz to 7 kHz, thus with a significant frequency at 5 kHz. The beam forming effect results in a higher amplification in 45° direction than in the 135° direction. Nevertheless, also for applications of the device, where at least not speech understanding is predominantly addressed, the 5 kHz frequency is significant for pronounced pinna directivity effect.
As was mentioned above, the transfer characteristics as exemplified by FIGS. 2 to 5 are transfer characteristics at respective acoustical signal frequencies of an individual's ear per se.
When applying to such individual a microphone arrangement not in the ear canal, e.g. behind the pinna, as is customarily done by applying a behind-the-ear hearing aid device, the directivity characteristic of the pinna becomes moot, whereas the HRTF-based characteristic as of the FIGS. 2 to 4 is still effective.
Thus, an intrinsically omnidirectional beam former with microphone arranged not in the ear canal, thus especially behind the pinna will establish a transfer characteristic with substantially equal amplification symmetrical to the 90° direction. The pinna-caused beam forming characteristic with an attenuation of signals impinging from 45° relative to such signals from 135° which according to FIG. 4 is about +6 dB is lost. It results therefrom that whenever the hearing device enabled or controllably switched in omnidirectional mode will not establish for a transfer characteristics which accords to natural beam forming of the ear at frequencies above 2 kHz and, with respect to speech understanding, in the relevant frequency band up to about 7 kHz, but will establish as shown in FIG. 5 by dashed lines a transfer characteristic as if no pinna was present.